This invention relates to a discharge lamp operation apparatus for staring and operating discharge lamp at a high frequency.
Recently, in the discharge lamp operation apparatus, attempts have been made to operate the discharge lamp using a high frequency inverter which is a switching circuit composed of a semiconductor device, in order to increase efficiency and reduce size and decrease weight. More recently, in particular, a compact fluorescent lamp is developed, and a smaller size has been demanded of the discharge lamp operation apparatus using a high frequency inverter. First the conventional discharge lamp operation apparatus is explained below while referring to the drawings.
FIG. 1 is a circuit diagram of a conventional discharge lamp operation apparatus, in which numeral 1 denotes a power supply circuit, 2 is an inverter circuit, and element 3 is a discharge lamp load circuit. The power supply circuit 1 rectifies and smooths an alternating-current power supply source 4 in a rectifying circuit 5, and delivers a DC output. Describing a DC inverter circuit 2, the emitter of a transistor 6 is connected to the negative side of the rectifying circuit 5, and a capacitor 7 for resonance is connected between the collector and the positive side of the rectifying circuit 5. One of the terminals of discharge lamp load circuit 3 is connected to the collector of the transistor 6, and the other one is connected to the positive side of the rectifying circuit 5 by way of a choke coil 8. This choke coil 8 is provided with a feedback winding 9, and one of its terminals is connected to the negative side of the rectifying circuit 5 while the other one is connected to the base of the transistor 6 through a series circuit consisting of an inductor 10 which is the driving inductor and a capacitor 11 which is the driving capacitor. A starting resistance 12 is connected between the base and the positive side of the rectifying circuit 5, and in order to feed a base current to the transistor 6 after start-up, a diode 13 and a resistance 14 are connected in series from the emitter to the base. The discharge lamp load circuit 3 comprises a fluorescent lamp 15 series assembly of a diode 18 and constant voltage switching device 19 connected as starting circuit 17 in parallel with the lamp 15 so as to be in forward direction with respect to the direct-current power supply between the non-feeding side terminals of its filament electrodes 16a and 16b, and capacitor 20 connected in parallel with the lamp 15 for stabilizing the oscillation of the inverter circuit 2 during its operation.
The operation of the thus composed conventional discharge lamp operation apparatus is explained below.
When the AC power supply 4 supplies power, it is rectified and smoothed in the rectifying circuit 5, and the current passes through the resistance 12 to flow into the base of the transistor 6 to turn on the transistor 6. Into the collector of the transistor 6, an electric current flows from the positive side of the rectifying circuit 5 through the choke coil 8, filament electrode 16a of fluorescent lamp 15, diode 18 and constant voltage switching element 19 of the starting circuit 17, and filament electrode 16b. On the other hand, the feedback winding 9 is connected to the choke coil 8, and through the series assembly of the inductor 10 and capacitor 11, the current flowing in the choke coil 8 is positively fed back to the base of the transistor 6. The circuit composes a resonance, circuit together with the capacitor 7 and the inductance of the choke coil 8, thereby starting oscillation.
As for the oscillation load current, since the series assembly of diode 18 and constant voltage switching element 19 are connected in parallel to the non-feeding side of the fluorescent lamp 15 in the forward direction, the filament electrodes 16a, 16b of the fluorescent lamp 15 begin to be preheated by the current flowing from the positive side of the rectifying circuit 5 to the collector when the transistor 6 is turned on. On the other hand, as for the resonance voltage generated when the transistor 6 is turned off, since the diode 18 connected to the non-feeding side of the fluorescent lamp 15 is in a reverse direction, the current cannot flow, and a high voltage is applied between the filaments 16a, 16b of the fluorescent lamp 15, and the fluorescent lamp 15 is discharged, and its discharge current flows into the positive side of the rectifying circuit 5 from the collector. Thereafter, repeating preheating and discharge alternately, the filament electrodes 16a, 16b of the fluorescent lamp 15 are preheated, and when preheated as specified, the fluorescent lamp 15 is ignited. When ignited, the voltage between the filament electrodes 16a, 16b becomes the lamp voltage for the fluorescent lamp 15. Since the breakover voltage of the constant voltage switching element 19 is set slightly higher than the lamp voltage of the fluorescent lamp 15, after the fluorescent lamp 15 is ignited, no current flows in the constant voltage switching element 19.
In such composition, however, since the circuit is composed to operate the choke coil 8 as the resonant inductor of the inverter, while the fluorescent lamp 15 is ignited, the DC component flowing through the transistor 6 is superposed on the high frequency current, and the current as shown in FIG. 2 flows into the discharge path of the fluorescent lamp 15.
It does not matter that the current flowing in the discharge path of the fluorescent lamp 15 is a symmetric alternating current, but when it is ignited for a long period by passing an alternating current containing DC components as in the above composition when the ambient temperature is low, the mercury vapor is concentrated on one electrode, and emission ununiformity occurs in the fluorescent lamp 15, so that the luminous flux is lowered. This is the so-called cataphoresis phenomenon.
Furthermore, when starting up the fluorescent lamp 10, the energy for starting up is insufficient, and if the series resonance of the capacitor 20 and choke coil 8 is utilized in order to improve it, the impedance of the fluorescent lamp 10 varies largely, suddenly and irregularly in the abnormal state upon starting up or in the terminal phase of the service life. As a result, not only the resonance occurring between the series resonance circuit of capacitor 20 and choke coil 8 and the capacitor 7 increases, but also the inductance value between the driving capacitor 11 and inductance 10 varies, and the storage time of the transistor 11 varies, thereby inducing periodic deviation and abnormal oscillation, and finally breakdown of the circuit.
Moreover, a large voltage is suddenly applied on the fluorescent lamp 15, which can shorten the life service of the fluorescent lamp 15.